SE2018_Lec 16_ Architectural Design

1
‫ر‬َ‫ـد‬ْ‫ق‬‫ِـ‬‫ن‬،،،‫لما‬‫اننا‬ ‫نصدق‬ْْ‫ق‬ِ‫ن‬‫ر‬َ‫د‬

2
 The software architecture of a program or computing
system is the structure (or structures) of the system,
including:
 Dividing software into subsystems.
 Deciding how these will interact.
 Determining their interfaces.
• The architecture is the core of the design, so all software
engineers need to understand it.
• The architecture will often constrain the overall efficiency,
reusability and maintainability of the system

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 Why you need to develop an architectural model:
 To enable everyone to better understand the system
 To allow people to work on individual pieces of the system
in isolation
 To prepare for extension of the system
 To facilitate reuse and reusability

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 An architecture is an abstraction of a system that
suppresses details of components that do not affect how
they:
 Use other components
 Are used by other components
 Relate to other components
 Interact with other components

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 To ensure the maintainability and reliability of a system,
an architectural model must be designed to be stable.
 Being stable means that the new features can be easily
added with only small changes to the architecture.

6
aTruck aShip aAirplane theWarehouseCollecti on
theVehicleCollection
UML-A Generated Dependency Class:theRouter Dependency (1.0)
theStorage
aVehicle
availableVehicleCollection
UML-A Generated Association Class:theVehicleCollection Generalization (1.0)UML-A Generated Association Class:theVehicleCollection Generalization (1.0)UML-A Generated Association Class:theVehicleCollection Generalization (1.0)UML-A Generated As sociationC lass:theVehicleC ollec tion Generalization (1.0)UML-A Generated Association Class:theVehicleCollection Generalization (1.0)UML-A Generated Association Class:theVehicleCollection Generalization (1.0)UML-A Generated Association Class:theVehicleCollection Generalization (1.0)UML-A Generated Association Class:theVehicleCollection Generalization (1.0)UML- A Generated Ass ociati onCl ass:theVehi cleCollection Generali zation (1.0)UML-A Generated Association Class:theVehicleCollection Generalization (1.0)
availableGoods
aPort
aPortC ollec tion
aSurplus aDifficiency
theTimeNeeded
theGoods
UML-A Generated Association Class:aWarehouse Association (0.5)
UML-A Generated Association Class:availableGoods Association (0.5)
aRouteCollection
UML-A Generated Association Class:aRoute Association (0.25)UML-A Generated Association Class:aRoute Association (0.25)UML-A Generated Association Class:aRoute Association (0.25)
UML-A Generated Association Class:aRoute Association (0.25)
UML-A Generated Dependency Class:theRouter Dependency (0.5)UML-A Generated Dependency Class:theRouter Dependency (1.0)
theAWT
aVehiceDialog aWarehouseDialog aPortDialog aRouterDialog
aWarehouse
UML-A Generated AssUML-A Generated Association Class:aDifficiency Associ
UML-A Generated Association Class:aDifficiency AssociatioUML-A Generated Association Class:aDifficiency Association (1.0
UML-A Generated Association Class:aDifficiency AU ML-A Generated AssociationClaUML-A Generated AssociatioUML-A Generated Association Class:aDifficieU ML-A Generated AssociationClass:aDUML-A Generat
UML-A GeneraUML-A Generated As
aLocation
UML-A Generated Association Class:aNavPoint Association (0.5)
aNavPoint
UML-A Generated Association Class:aWarehouse Association (0.5)UML-A Generated Association Class:aWarehouse Association (0.5)
UML-A Generated Association Class:aRoute Association (0.5)
aRoute
UML-A Generated Dependency C lass :aRouteCol lection Ass ociation (0.25)
UML-A Generated Association Class:aNavPoint Association (1.0)UML-A Generated Association Class:aNavPoint Association (0.5)
UML-A Generated Dependency Class:aRouteCollection Association (0.5)
UML-A Generated Association Class:aNavPoint Association (1.0)UML-A Generated Association Class:aNavPoint Association (1.0)UML-A Generated Association Class:aNavPoint Association (1.0)UML-A Generated Association Class:aNavPoint Association (1.0)UML-A Generated Association Class:aNavPoint Association (1
UML-A Generated Dependency Class:theRouter Association (0.25)
theCargoRouter
UML-A Generated As sociationC lass:theWarehouseCollection Dependency ( 0.25)
UML-A Generated Association Class:theRouter Association (0.25)
UML-A Generated Association Class:theRouter A
theRouter
UML-A Generated Association Class:theWarehouseCollection Dependency (0.5)
UML-A Generated Dependency Class :aRoUML-A Generated Association Clas s:theWarehouseC
UML-A Generated Association Class:theVehicleCollection Dependency (0.5)UML-A Generated Association Class:availableVehicleCollection Dependency (0.5)
UML- A Generated Generaliz ation Class :avail ableVehicleCollection Dependenc y (1.0)
UML-A Generated Dependency Class:theWarehouseCollection Dependency (1.0)
UML-A Generated Dependency Class:theRouter Association (1.0)UML-A Generated Dependency Class:theRouter Association (1.0)
This is a simple
software system!

8
Independent Components
Communicating
Processes
Event Systems
Client/Server Peer-to-Peer
Implicit
Invocation
Explicit
Invocation
Data Flow
Batch Sequential Pipe & Filter
Virtual Machine
Interpreter Rule-Based
System
Data-Centered
Repository Blackboard
Call and Return
Main Program
and Subroutine
Object
OrientedLayered
Remote Procedure Call

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Shared Data
Client A Client B Client C
Client D Client E Client F

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 Has goal of achieving the quality of integrability of data.
 The term refers to systems in which the access and update
of a widely accessed data store is their primary goal.
 Basically, it is nothing more than a centralized data store
that communicates with a number of clients.
 Important for this styles are three protocols:
communication, data definition and data manipulation
protocol
 A client runs on an independent thread of control.
 The means of communication distinguishes the two
subtypes: repository and blackboard
 Repository: a client sends a request to the system to perform
a necessary action (e.g. insert data)
 Blackboard: the system sends notification and data to
subscribers when data of interest changes, and is thus active

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 Ensures data integrity
 Reliable, secure, easy to Backup, testability guaranteed
 Clients independent on the system: performance and
usability on the client side is good
 Problems with scalability
 Solutions: shared repositories, replication but this
increases complexity

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 One of the most well-known examples of the data-
centered architecture, is a database architecture
 E.g. in RDBMS a set of related tables with fields, data
types, keys, ...
 Clients use data manipulation protocol to work with the
data
 E.g. SQL for inserting, selecting, deleting data, ...
 Depending on where clients are situated communication
protocol might be
 An inner-process communication
 Communication between components on the same machine
 Communication over network, e.g. LAN, Internet, etc.

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 Has the goal of achieving the quality of reuse and
modifiability.
 The data-flow style is characterized by viewing the system
as a series of transformations on successive pieces of input
data.
 Data enter the system and then flows through the
components one at a time until
 Finally, the data is assigned to some final destination
(output or a data store).
 The architecture is very flexible.
 Almost all the components could be removed.
 Components could be replaced.
 New components could be inserted.
 Certain components could be reordered.

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Validate Sort Update Report

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 Batch sequential style
 The processing steps are independent components
 Each step runs to completion before the next step begins
 Pipe-and-filter style
 Emphasizes the incremental transformation of data by
successive components
 The filters incrementally transform the data (entering and
exiting via streams)
 The pipes are stateless and simply exist to move data
between filters
 Note That: It is easily made into a parallel or distributed
execution in order to enhance system performance

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 Data flows through pipes: communication channels
between filters
 Processing units: filters
 Filters do not know anything about other filters
 Modularity, maintainability is good
 Data flows in streams: good for processing of images,
audio, video, ...
 Depending on where the filters reside different
execution architectures
 E.g. same process: filters run within threads
 E.g. same machine: filters run within own processes
 E.g. network: pipes use the networking architecture

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 Pipe and Filter Example:
 Traditional Compilers: Compilation phases are pipelined,
though the phases are not always incremental. The phases
in the pipeline include
• lexical analysis + parsing + semantic analysis + code
generation

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Main module
Subroutine A
Subroutine B
Subroutine A-1 Subroutine A-2
Physical layer
Data layer
Network layer
Transport layer
Application layer Class WClass V
Class X
Class Z
Class Y

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 Has the goal of modifiability and scalability
 Has been the dominant architecture since the start of
software development
 Main program and subroutine style
 Decomposes a program hierarchically into small pieces (i.e.,
modules)
 Typically has a single thread of control that travels through
various components in the hierarchy
 Remote procedure call style
 Consists of main program and subroutine style of system that is
decomposed into parts that are resident on computers
connected via a network
 Strives to increase performance by distributing the computations
and taking advantage of multiple processors
 Incurs a finite communication time between subroutine call and
response.

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 Object-oriented or abstract data type system
 Emphasizes the bundling of data and how to manipulate and
access data
 Keeps the internal data representation hidden and allows access
to the object only through provided operations
 Permits inheritance and polymorphism
 Layered system
 Assigns components to layers in order to control inter-
component interaction
 Only allows a layer to communicate with its immediate neighbor

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 Layering: the structure of the system is organized into
set of layers
 Each layer in on the top of another layer, each layer
communicates only with the layer immediately below it.
 The higher layer sees the lower layer as a set of services.
 Well-defined interfaces between layers
 Reduces complexity,
 improves modularity,
 reusability,
 maintainability

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 Typically organized into layers
 Successive layers provide more sophisticated services to
the layers above them
 Hardware services, kernel services, system services, UI
services

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 A virtual machine implements an
instruction set for an imaginary
machine
 Often virtual machines are the
underplaying mechanism by which
a programming language is
executed
 E.g. Java virtual machine, different
interpreters
• Specifies an interface between
compiler and a real machine
 From conceptual point of view very
similar to OS
 Improves portability

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 A common example of layered architecture is a network
protocol stack
 E.g. TCP/IP protocol stack - Four layers
 The lowest layer: handles communication between two
computers
 The internet layer: handles routing of packets across the
network
 The transport layer: guarantees that packets are error-
free and received in the same order as sent
 The application layer: supports application-specific
protocols
• E.g. HTTP, SMTP, FTP, ...

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 Basic concept:
 The client uses a service
 The server provides a service
 Typically connected via a network
 Clients are independent from each other
 There is at least one component that has the role of
client, initiating connections in order to obtain some
service.

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 Conceptually simple
 Clear separation of responsibilities, help testability
 Good scalability (if stateless)
 Excellent scalability (if server can be scaled out)
 Good for security, as data can be held at the server with
restricted access
 Risk of bad performance, if the communication between
client and server is slow, or has a high latency
 Need to develop/agree on a protocol between client and
server
 For stateful, centralized servers scalability is limited

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 The server is no longer in the organizations network, but
somewhere in the Internet
 Example: cloud services by Salesforce, Google,
Microsoft
 Scalability, security, reliability is expected to be
handled by a specialized team
 Needs a working Internet connection

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 Separation between client and server is removed
 Each client is a server at the same time, called peer
 The goal is to distribute the processing or data among many
peers
 No central administration or coordination
29

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 Example: Skype uses a peer-to-peer protocol, but also
uses super-nodes and a central login servers
 Adv.
 Good for scalability
 Good for reliability, as data can be replicated over peer
 No single point of failure
 Disadv.
 Quality of service is not deterministic, cannot be guaranteed
 Very complex, hard to maintain and test

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 The N-tier architecture is the modern client-server
architecture
 Originated with business applications
 Through the popularity of the Web today typically
related with Web applications
 Conceptually separate architecture into:
 Presentation,
 Application, and
 Data storage layers.

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 Clients are typically rich
(ui + app-logic + communication)
 Servers store data
 Each client runs a complete application
 Drawbacks: each client has to know how to
communicate with all data servers
 Scalability is compromised because client are tightly
coupled with servers

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 Evolved from 2-tier architectures to solve
their drawbacks
 A third tier is inserted between clients
and data servers
 Application or business logic tier: middle
tier.
 Typically middle tier is on the server side
(but recently might be split between the
server and the client)
 Scalability improved because clients are
thinner.
 Thin clients have no knowledge on
application
 A rich client contains full knowledge of
application

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 Suitable for applications in which a central issue is
identifying and protecting related bodies of data.
 Data representations and their associated operations are
encapsulated in an abstract data type.
 Components: are objects.
 Connectors: are function and procedure invocations
(methods).

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 Object-Oriented Invariants
 The data representation is hidden from other objects.
 Advantages
 it is possible to change the implementation without
affecting those clients.
 Can design systems as collections of autonomous
interacting agents.
 Disadvantages
 In order for one object to interact with another object (via
a method invocation) the first object must know the
identity of the second object
 Objects cause side effect problems:
• E.g., A and B both use object C, then B’s effects on C look
like unexpected side effects to A.

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 Suitable for applications that involve loosely-coupled
collection of components, each of which carries out
some operation and may in the process enable other
operations.
 Instead of invoking a procedure directly ...
 A component can announce (or broadcast) one or more
events.
 Other components in the system can register an interest in
an event by associating a procedure with the event.
 When an event is announced, the broadcasting system
(connector) itself invokes all of the procedures that have
been registered for the event.

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 Advantages:
 Provides strong support for reuse.
 Eases system evolution
 Disadvantages:
 When a component announces an event:
• it has no idea what other components will respond to it,
• it cannot rely on the order in which the responses are
invoked
• it cannot know when responses are finished
 Twitter, Google+

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 Remote invocation architectures involve distributed
processing components
 Typically, a client component invokes a method
(function) on a remote component
 E.g. Web services
 Advantages:
 increased performance through distributed computation
 Disadvantages:
 tightly coupling of components
 increases communication overhead

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 Transparently distribute
aspects of the software
system to different nodes
 An object can call
methods of another
object without knowing
that this object is
remotely located.

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 Systems are seldom built from a single architectural
style
 Three kinds of heterogeneity:
 Locationally heterogeneous
• The drawing of the architecture reveals different styles in
different areas (e.g., a branch of a call-and-return system
may have a shared repository)
 Hierarchically heterogeneous
• A component of one style, when decomposed, is structured
according to the rules of a different style
 Simultaneously heterogeneous
• Two or more architectural styles may both be appropriate
descriptions for the style used by a computer-based system

42
‫ر‬َ‫ـد‬ْ‫ق‬‫ِـ‬‫ن‬،،،‫لما‬‫اننا‬ ‫نصدق‬ْْ‫ق‬ِ‫ن‬‫ر‬َ‫د‬

43
 Are concerned with the control flow between sub-
systems.
 Centralized control
 One sub-system has overall responsibility for control and
starts and stops other sub-systems
 Event-based control
 Each sub-system can respond to externally generated
events from other sub-systems or the system’s
environment

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 A control sub-system takes responsibility for managing
the execution of other sub-systems
 Call-return model
 Top-down subroutine model
 Control starts at the top of a subroutine hierarchy and
moves downwards.
 Applicable to sequential systems
 Manager model
 One system component controls the stopping, starting
and coordination of other system processes.
 Applicable to concurrent systems.

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Routine 1.2Routine 1.1 Routine 3.2Routine 3.1
Routine 2 Routine 3Routine 1
Main
program

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System
controller
User
interface
Fault
handler
Computation
processes
Actuator
processes
Sensor
processes

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 Driven by externally generated events where the
timing of the event is outside the control of the sub-
systems which process the event
 Two principal event-driven models:
 Broadcast models. An event is broadcast to all sub-
systems. Any sub-system which can handle the event
may do so
 Interrupt-driven models. Used in real-time systems
where interrupts are detected by an interrupt handler
and passed to some other component for processing

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Sub-system
1
Event and message handler
Sub-system
2
Sub-system
3
Sub-system
4

49
Handler
1
Handler
2
Handler
3
Handler
4
Process
1
Process
2
Process
3
Process
4
Interrupts
Interrupt
vector

SE2018_Lec 16_ Architectural Design

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SE2018_Lec 16_ Architectural Design